Corrosion studies on aluminium reinforced with uncoated and coated carbon fibres

Abstract

Knowing materials behaviour under conditions of electrochemical attack is an important necessity for engineering applications, especially in marine environments. The considerable deterioration of the electrochemical corrosion resistance which results from the reinforcement of aluminium with carbon fibres demands preventative measures. In order to retain usual life times of applications, suitable corrosion protection has to be implemented. The present study deals with an internal corrosion protection process resulting from the use of suitable fibre coatings, viz. a pyrolytic carbon (pyC) coating and a nickel fibre coating. However, neither the ceramic pyC coating nor the metallic nickel coating effect an improvement of the corrosion resistance in aqueous solutions of 3.5 wt% NaCl or 3.5 wt% Na 2SO 4. Although, the simple immersion test confirmed the lowest anodic matrix dissolution for metal-matrix composites (MMCs) formerly containing nickel-coated fibres, the polarisation of the MMCs to more positive potentials causes greatly accelerated electrochemical dissolution. Furthermore, the observed segregation of elemental nickel to the sample surface after the immersion tests in both electrolytes indicate additional susceptibility to galvanic coupling. MMCs reinforced with carbon fibres obtaining a pyrolytic carbon coating show evidence of strongly accelerated electrochemical corrosion. Because the microstructure and the surface area of the pyrolytic carbon differs from that of the carbon fibres, a probable reason for the enhanced corrosion susceptibility may be the higher reactivity of the pyC coating. This result stresses the importance of taking into account the features of the carbon of the reinforcing component and to distinguish between different carbon types. Special emphasis should be placed on the processing of pyC coatings in order to generate pyC coatings with few active sites and low reactivity.